socket(2) — Linux manual page


SOCKET(2)                 Linux Programmer's Manual                SOCKET(2)

NAME         top

       socket - create an endpoint for communication

SYNOPSIS         top

       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       int socket(int domain, int type, int protocol);

DESCRIPTION         top

       socket() creates an endpoint for communication and returns a file
       descriptor that refers to that endpoint.  The file descriptor
       returned by a successful call will be the lowest-numbered file
       descriptor not currently open for the process.

       The domain argument specifies a communication domain; this selects
       the protocol family which will be used for communication.  These
       families are defined in <sys/socket.h>.  The formats currently
       understood by the Linux kernel include:

       Name         Purpose                                    Man page
       AF_UNIX      Local communication                        unix(7)
       AF_LOCAL     Synonym for AF_UNIX
       AF_INET      IPv4 Internet protocols                    ip(7)
       AF_AX25      Amateur radio AX.25 protocol               ax25(4)
       AF_IPX       IPX - Novell protocols
       AF_APPLETALK AppleTalk                                  ddp(7)
       AF_X25       ITU-T X.25 / ISO-8208 protocol             x25(7)
       AF_INET6     IPv6 Internet protocols                    ipv6(7)
       AF_DECnet    DECet protocol sockets
       AF_KEY       Key management protocol, originally
                    developed for usage with IPsec
       AF_NETLINK   Kernel user interface device               netlink(7)
       AF_PACKET    Low-level packet interface                 packet(7)
       AF_RDS       Reliable Datagram Sockets (RDS) protocol   rds(7)
       AF_PPPOX     Generic PPP transport layer, for setting
                    up L2 tunnels (L2TP and PPPoE)
       AF_LLC       Logical link control (IEEE 802.2 LLC)
       AF_IB        InfiniBand native addressing
       AF_MPLS      Multiprotocol Label Switching
       AF_CAN       Controller Area Network automotive bus
       AF_TIPC      TIPC, "cluster domain sockets" protocol
       AF_BLUETOOTH Bluetooth low-level socket protocol
       AF_ALG       Interface to kernel crypto API
       AF_VSOCK     VSOCK (originally "VMWare VSockets")       vsock(7)
                    protocol for hypervisor-guest
       AF_KCM       KCM (kernel connection multiplexer)
       AF_XDP       XDP (express data path) interface

       Further details of the above address families, as well as information
       on several other address families, can be found in

       The socket has the indicated type, which specifies the communication
       semantics.  Currently defined types are:

       SOCK_STREAM     Provides sequenced, reliable, two-way, connection-
                       based byte streams.  An out-of-band data transmission
                       mechanism may be supported.

       SOCK_DGRAM      Supports datagrams (connectionless, unreliable
                       messages of a fixed maximum length).

       SOCK_SEQPACKET  Provides a sequenced, reliable, two-way connection-
                       based data transmission path for datagrams of fixed
                       maximum length; a consumer is required to read an
                       entire packet with each input system call.

       SOCK_RAW        Provides raw network protocol access.

       SOCK_RDM        Provides a reliable datagram layer that does not
                       guarantee ordering.

       SOCK_PACKET     Obsolete and should not be used in new programs; see

       Some socket types may not be implemented by all protocol families.

       Since Linux 2.6.27, the type argument serves a second purpose: in
       addition to specifying a socket type, it may include the bitwise OR
       of any of the following values, to modify the behavior of socket():

       SOCK_NONBLOCK   Set the O_NONBLOCK file status flag on the open file
                       description (see open(2)) referred to by the new file
                       descriptor.  Using this flag saves extra calls to
                       fcntl(2) to achieve the same result.

       SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new
                       file descriptor.  See the description of the
                       O_CLOEXEC flag in open(2) for reasons why this may be

       The protocol specifies a particular protocol to be used with the
       socket.  Normally only a single protocol exists to support a
       particular socket type within a given protocol family, in which case
       protocol can be specified as 0.  However, it is possible that many
       protocols may exist, in which case a particular protocol must be
       specified in this manner.  The protocol number to use is specific to
       the “communication domain” in which communication is to take place;
       see protocols(5).  See getprotoent(3) on how to map protocol name
       strings to protocol numbers.

       Sockets of type SOCK_STREAM are full-duplex byte streams.  They do
       not preserve record boundaries.  A stream socket must be in a
       connected state before any data may be sent or received on it.  A
       connection to another socket is created with a connect(2) call.  Once
       connected, data may be transferred using read(2) and write(2) calls
       or some variant of the send(2) and recv(2) calls.  When a session has
       been completed a close(2) may be performed.  Out-of-band data may
       also be transmitted as described in send(2) and received as described
       in recv(2).

       The communications protocols which implement a SOCK_STREAM ensure
       that data is not lost or duplicated.  If a piece of data for which
       the peer protocol has buffer space cannot be successfully transmitted
       within a reasonable length of time, then the connection is considered
       to be dead.  When SO_KEEPALIVE is enabled on the socket the protocol
       checks in a protocol-specific manner if the other end is still alive.
       A SIGPIPE signal is raised if a process sends or receives on a broken
       stream; this causes naive processes, which do not handle the signal,
       to exit.  SOCK_SEQPACKET sockets employ the same system calls as
       SOCK_STREAM sockets.  The only difference is that read(2) calls will
       return only the amount of data requested, and any data remaining in
       the arriving packet will be discarded.  Also all message boundaries
       in incoming datagrams are preserved.

       SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to
       correspondents named in sendto(2) calls.  Datagrams are generally
       received with recvfrom(2), which returns the next datagram along with
       the address of its sender.

       SOCK_PACKET is an obsolete socket type to receive raw packets
       directly from the device driver.  Use packet(7) instead.

       An fcntl(2) F_SETOWN operation can be used to specify a process or
       process group to receive a SIGURG signal when the out-of-band data
       arrives or SIGPIPE signal when a SOCK_STREAM connection breaks
       unexpectedly.  This operation may also be used to set the process or
       process group that receives the I/O and asynchronous notification of
       I/O events via SIGIO.  Using F_SETOWN is equivalent to an ioctl(2)
       call with the FIOSETOWN or SIOCSPGRP argument.

       When the network signals an error condition to the protocol module
       (e.g., using an ICMP message for IP) the pending error flag is set
       for the socket.  The next operation on this socket will return the
       error code of the pending error.  For some protocols it is possible
       to enable a per-socket error queue to retrieve detailed information
       about the error; see IP_RECVERR in ip(7).

       The operation of sockets is controlled by socket level options.
       These options are defined in <sys/socket.h>.  The functions
       setsockopt(2) and getsockopt(2) are used to set and get options.

RETURN VALUE         top

       On success, a file descriptor for the new socket is returned.  On
       error, -1 is returned, and errno is set appropriately.

ERRORS         top

       EACCES Permission to create a socket of the specified type and/or
              protocol is denied.

              The implementation does not support the specified address

       EINVAL Unknown protocol, or protocol family not available.

       EINVAL Invalid flags in type.

       EMFILE The per-process limit on the number of open file descriptors
              has been reached.

       ENFILE The system-wide limit on the total number of open files has
              been reached.

              Insufficient memory is available.  The socket cannot be
              created until sufficient resources are freed.

              The protocol type or the specified protocol is not supported
              within this domain.

       Other errors may be generated by the underlying protocol modules.

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, 4.4BSD.

       The SOCK_NONBLOCK and SOCK_CLOEXEC flags are Linux-specific.

       socket() appeared in 4.2BSD.  It is generally portable to/from non-
       BSD systems supporting clones of the BSD socket layer (including
       System V variants).

NOTES         top

       POSIX.1 does not require the inclusion of <sys/types.h>, and this
       header file is not required on Linux.  However, some historical (BSD)
       implementations required this header file, and portable applications
       are probably wise to include it.

       The manifest constants used under 4.x BSD for protocol families are
       PF_UNIX, PF_INET, and so on, while AF_UNIX, AF_INET, and so on are
       used for address families.  However, already the BSD man page
       promises: "The protocol family generally is the same as the address
       family", and subsequent standards use AF_* everywhere.

EXAMPLES         top

       An example of the use of socket() is shown in getaddrinfo(3).

SEE ALSO         top

       accept(2), bind(2), close(2), connect(2), fcntl(2), getpeername(2),
       getsockname(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2),
       select(2), send(2), shutdown(2), socketpair(2), write(2),
       getprotoent(3), address_families(7), ip(7), socket(7), tcp(7),
       udp(7), unix(7)

       “An Introductory 4.3BSD Interprocess Communication Tutorial” and “BSD
       Interprocess Communication Tutorial”, reprinted in UNIX Programmer's
       Supplementary Documents Volume 1.

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-06-09                        SOCKET(2)

Pages that refer to this page: accept(2)accept4(2)bind(2)bpf(2)connect(2)creat(2)fcntl(2)fcntl64(2)getsockname(2)getsockopt(2)listen(2)mknod(2)mknodat(2)open(2)openat(2)recv(2)recvfrom(2)recvmmsg(2)recvmsg(2)send(2)sendfile(2)sendfile64(2)sendmmsg(2)sendmsg(2)sendto(2)setsockopt(2)shutdown(2)socketcall(2)socketpair(2)syscalls(2)audit_open(3)freeaddrinfo(3)freeifaddrs(3)gai_strerror(3)getaddrinfo(3)getifaddrs(3)getnameinfo(3)if_freenameindex(3)if_indextoname(3)if_nameindex(3)if_nametoindex(3)pcap_set_protocol_linux(3pcap)pmda(3)PMDA(3)pmdaconnect(3)pmdaConnect(3)systemd.exec(5)address_families(7)ddp(7)ip(7)packet(7)raw(7)sctp(7)signal-safety(7)socket(7)tcp(7)unix(7)vsock(7)x25(7)